Of late, Pb-Ca alloy has been used for plate grids because it is more convenient for maintenance of a lead acid storage battery than Pb-Sb alloy. Meanwhile, in order to automatically produce plate grids to improve the efficiency at which the plate grids are produced and to delete the conventional operation of casting plate grids under unfavorable circumstances, it has been developed to expand or punch a lead alloy strip to form plate grids. In the United States, storage batteries for cars have been commercially available which have the expanded plate grids provided therein. The expanded plate grids can be more efficiently produced than cast plate grids because the former are produced by succesively expanding a lead alloy strip while supplied from a roll of lead alloy strip. Since the cast plate grids are cast in a casting mould in which only one or two plate grids can be cast, they are produced at a lower efficiency. On the other hand, the plates having expanded grids can be obtained by a flow production of supplying the lead alloy strip, expanding it, pasting the thus produced mesh-like strip, drying it and cutting it to form individual grids. This allows the plate grids to be automatically produced.
However, a storage battery having expanded plate grids of Pb-Ca alloy has the disadvantage that the storage battery has a lower capacity than a storage battery having cast plate grids of Pb-Sb alloy. Various factors will be considered to cause the capacity of the storage battery having the expanded plate grids to be lowered, which is referred to later as cycle deterioration, but tight adhesion between the plate grids and active materials is considered to be one of the important factors. We believe that improvement on tight adhesion between the plate grids and the active materials contributes largely to prevention of the cycle deterioration.
It is known from various literatures that antimony of Pb-Sb alloy plate grids serves largely to improve the adhesion between the plate grids and the active materials. But, calcium of Pb-Ca alloy plate grids cannot expect such an improvement of the adhesion between the plate grids and the active materials.
In addition to such a disadvantage of Pb-Ca alloy of which the plate grids are made, the expanded plate grids have such a disadvantage that they have surfaces flatter than the cast plate grids. The cast plate grids have surfaces substantially roughened by transferring to the surfaces of the plate grids the contour of fine powder of cork which is sprayed against a surface of a casting mould and referred to as a slip agent (heat insulating agent and removal agent). Thus, the active materials are mechanically meshed with the plate grids by the roughness of the plate grids and the surface area of the plate grids increases because of the roughness of the grid surfaces. These assure the substantial adhesion of the active materials with the plate grids. On the other hand, the expanded plate grids cannot expect such a mechanical mesh of the active materials with the plate grids because they are formed by rolling the lead alloy strip. Furthermore, the expanded plate grids are so formed by cutting the expanded lead alloy strip by a predetermined length that the sectional faces of the plate grids are flatter than the surfaces of the cast plate grids although it has a microscopical roughness. Thus, it will be noted that the surface area of the expanded plate grids is smaller than that of the cast plate grids. On casting ingot for making the lead alloy strip, a calcium oxide layer is possibly formed on the surface of the ingot. Thus, the calcium oxide layer is also covered on the surface of the expanded plate grids which are produced by rolling and working the lead alloy strip. This will cause the adhesion between active materials and the plate grids to be lowered.
Such a poor adhesion between the active materials and the plate grids has been experienced in our tests in which we can determine this by dropping the active materials out of the plate grids while the plate is being vibrated. In comparison of the cast plate grids of Pb-Ca alloy with the expanded plate grids of Pb-Ca alloy, it has been assured that the latter has a cycle deterioration more than the former. This suggests that the poor adhesion between the active materials and the plate grids causes the cycle deterioration to be larger.